PAS resin is one of representative engineering plastics, and has excellent physical properties such as heat resistance, chemicals resistance, flame resistance, and electrical insulating properties. PAS resin can be widely used for computer accessories, automobile accessories, coatings for parts contacting corrosive chemicals, and industrial fibers with chemical resistance. At present, only polyphenylene sulfide of PAS resins is commercially available.
The representative method of preparing PPS resin is a Macullum process, in which PPS resin is synthesized by polymerizing an aromatic dichloride compound and sulfides in a polar organic solvent and described in U.S. Pat. Nos. 2,513,188 and 2,583,941.

As seen from the reaction scheme, p-dichlorobenzene and sodium sulfide are polymerized in a polar organic solvent such as N-methyl pyrrolidone to produce PPS resin and NaCl as a by-product.
The PPS resin synthesized in the Macullum process has a narrow range of application due to a molecular weight of 10,000 to 40,000 and melt viscosity of 3000 Poise or lower, resulting in narrow applications and it cannot be applied with post-treatment. That is, to improve the melt viscosity of PPS resin, the synthesized PPS resin is further cured at a temperature of lower than the melting temperature (Tm) of PPS resin. The melt viscosity of PPS resin oxidation increases due to oxidation, crosslinking, and polymer chain extension in the curing step.
However, disadvantages of the Macullum process are as follows. First, the use of sulfides such as sodium sulfide produces a large amount of a by-product (metal salt). In the case of using sodium sulfide, the amount of produced by-product is 52 weight % with respect to the weight of the starting material, thereby resulting in difficulty in treating the by-product and a low yield of PPS resin. In addition, the by-product remains in PPS resin at several ppm to several thousands of ppm, and increases electrical conductivity, causes corrosion of machines, and problems in spinning fiber. Second, the Macullum process adopts a solution polymerization method, and thus produces PPS resin in a very fine powder form with a low apparent density, thereby causing disadvantages in transportation and manufacturing processes. Third, the brittleness of PPS resin increases in a curing process for improving the melt viscosity of PPS resin, and thus lowers the mechanical properties such as impact strength and causes the color of PPS to be dark.
There have been many suggestions to resolve such problems, and these include a composition and method for preparing PPS resin described in U.S. Pat. Nos. 4,746,758 and 4,786,713. In the composition and method, diiodo-aryl compounds and solid sulfur instead of dichloride compounds and sulfides are polymerized by being directly heated in the absence of a polar organic solvent.
The preparation method includes an iodination and polymerization step. The aryl compounds are reacted with iodine to obtain diiodo-aryl compounds in the iodination step, followed by polymerization of the diiodo-aryl compounds with solid sulfur over a nitro compound catalyst to produce PAS resin. Iodine generated in gas formed in the process is recovered and reused for the iodination process. The iodine is substantially a catalyst.
The method can resolve the problems of the conventional Macullum process. That is, because iodine is the by-product of the process and can be easily recovered, the electrical conductivity is not increased and the amount of iodine remaining in the final product is very low, and waste is reduced due to reuse of the recovered iodine. In addition, since an organic solvent is not used in the polymerization process, the final resin can be obtained in a pellet form, thereby avoiding the problems of the fine powder.
The PAS resin obtained in the process has a higher molecular weight than that of the Macullum process, and thus needs not be cured.
However, the composition and method for preparing PAS resin has some problems as follows. First, because residual iodine molecules are corrosive, even a small amount of iodine remaining in the final PAS resin can cause problems in manufacturing machines, and the dark color of iodine makes the resultant PAS resin dark. Second, as solid sulfur is used in the polymerization process, disulfide bonds included in PAS resin deteriorate the thermal properties of the resin. Third, not using the nitro compound catalyst makes the resin light, but deteriorates the thermal properties compared to when using a catalyst due to an increase in the disulfide bond content.